The present invention relates to an on die thermal sensor (ODTS) in a semiconductor memory device, and more particularly, to an ODTS capable of maintaining temperature sensitivity by not lowering voltage variation of a temperature voltage with respect to temperature variation under low power supply voltage circumstances.
A dynamic random access memory (DRAM) cell includes a transistor for operating as a switch and a capacitor for storing a charge, i.e., data. According to whether the capacitor stores the charge, i.e., whether a terminal voltage of the capacitor is high or low, a logic level of the data is determined as a high level or a low level.
Since the data is stored in the capacitor as an accumulated electrical charge form, there is no power consumption for the data storage ideally. However, since there occurs a leak current due to a PN junction of a metal oxide semiconductor (MOS) transistor, the stored initial charge may be discharged and, thus, the data may vanish.
To prevent data loss, the data stored in a memory cell is read and the read data is restored the memory cell by recharging the memory cell with a normal charge before the data vanishes. This operation should be periodically performed in order to maintain data.
The above-mentioned recharging operation is called a refresh operation and, generally, a control of the refresh operation is performed by a DRAM controller. Due to the refresh operation, refresh power is consumed. In case of a battery operated system which requires lower power consumption, reducing power consumption is very important and is a critical issue.
One method of reducing the power consumption for the refresh operation is changing a refresh period according to temperature. As the temperature decreases, a data holding time of the DRAM becomes longer. Therefore, by dividing a temperature range into several temperature regions and by lowering a frequency of a refresh clock at a relative low temperature region, power consumption is reduced.
Accordingly, a device for correctly sensing the temperature of the inside of the DRAM and for adjusting the refresh clock frequency is required.
As a semiconductor unit is highly integrated and operates at a high speed, a significant amount of heat is generated. The generated heat increases internal temperature of the semiconductor unit and, thus, can prevent the semiconductor unit from normally operating. The generated heat may cause a defect in the semiconductor unit.
Therefore, a device for correctly sensing the temperature of the semiconductor unit and for outputting the sensed temperature information is needed.